Method of hot rolling stainless steel



United rates atent O METHGD F HGT RGLLTNG STAINLESS STEEL Matti H.Paldrala, littsburgh, and- John L. Scarry, Homestead, Pa., assignors toUnited States Steel Corpera tion, a corporation of New Hersey NoDrawing. Application February 7, 1950, Serial No. 142,938

1 Claim. or. s0 ee This invention relates to a method of preparing astainless steel object for hot working and more particularly topreparing stainless steel ingots, blooms or billets for hot rolling. Inthe rolling of stainless steel ingots down to semi-finished stock suchas slabs or billets, seams are often formed due to checking and crackingduring rolling. These checks or cracks may be closed on further rolling,but the metal on the sides of the cracks will not unite.

These defects are most apparent in stainless steels having poor hotworking characteristics, that is, steels which lose ductility when theycool below tern eratures of approximately 1950 and 2000" F. andtherefore crack and check at this temperature. Austenitic stainlesssteels, such as those containing 25% chromium and nickel fall in thiscategory.

Prior to our invention it was necessary to take certain precautions whenrolling such steels. The ingot would be heated to a temperature of about240 i and partially reduced. it was then returned to the soaking pit andreheated for two or three hours to raise the temperature above that atwhich the ingot would crack or check after which it was again partiallyreduced. After the second reduction the bloom would be cooled andconditioned by grinding or scarftng. It would then be returned to thesoaking pit and reheated for approximately 20 to hours after which itwould be rolled to the desired size. In many instances additionalreheating and conditioning steps were required. in spite of all theseprecautions a certain amount of checking and cracking occurred in thefinished slab and in many instances the cracking would be so ba that theslab would have to be scrapped and returned to the electric furnace.

It is therefore an object of our invention to provide a method ofpreparing stainless steel ingots for rolling which will cause anincrease in the average yield from the ingots.

Another object is to provide such a method which will result ineliminating or greatly reducing the cracking and checking whichordinarily occurs when hot reducing stainless steel.

Still another object is to provide such a method the! will maintain theproper temperatures which will permit the ingot to be rolled to sizewithout reheating and intermediate conditioning.

These and other objects will be more apparent after referring to thefollowing description.

We have found that by putting a coating of ceramic refractory on theingot prior to rolling, the ingot can be rolled through a plurality ofpasses directly to size without cracking or checking to any extentinasmuch as the coating is suficient to prevent an objectionable drop intemperature. The process is preferably carried out as follows:

The ingot is first heated to a temperature between 300 and 1000 F.(preferably between 500 and 800 F.) and a slurry of a ceramic refractoryis applied to the partially heated ingot. in order for the coating tostick properly to the ingot, the surface thereof should be free of loosescale. The process is therefore best adapted for those stainless steelswhich are relatively scaling resistant. The refractory is preferably asilicate or mixture of silicates. Refractories which have been used withsuccess include "ice a pyrophylite base mixture consisting of betweenapproximately and pyrophylite, between approximately 1% and 5%bentonite, 10% and 25% plastic fire clay, and the balance substantiallyall sodium silicate; a cold setting cement consisting of burnt clay, rawclay and sodium silicate, and a mixture of crushed silica brick and fireclay. hi the above mixtures the melting point of the pyrophylite issufi'icientiy high that the same remains solid at the hot rollingtemperature and the sodium silicate and fire clay act as binders whichare effective to hold the pyrophylite at both high and low temperatures.Ordinarily from 2% to 25% of binder may be used, but it is preferred touse not more than 15%. in place of the binders mentioned, other wellknown binders may be used. For example, organic binders, such as SulfitePitch may be used. Sufficient liquid is mixed with the refractory toform a slurry of the proper consistency to spray on the ingot at a lowpressure since this appears to be the easiest manner of application atpresent. The slurry may be applied at either higher or lowertemperatures than those specified, but the specified temperatures arebest for practical purposes. A coating thickness sufiicient to affordthe insulation required is used and a thickness between A; and /2 inchis preferred, but a thickness of /s inch is sufficient in manyinstances. The coating thickness could also be increased to one inchwithout detrimental effect. but above this thickness there is danger ofthe coating falling off. A thicker coating should be plied adjacent thecorners than on the remaining s ce since there is a tendency for thecoating to be knocked oi? the corners during the rolling operation. Thecoating is allowed to set and the coated ingot is then charged into thesoaking pit and heated to the working temperature which is approximately2400 F., after which it rolled to size through a plurality of passes.The coating is readily removed at room temperature by brushing.

While one embodiment of our invention has been described it will beapparent that other adaptations and modifications may be made withoutdeparting from the scope of the following claim.

We claim:

The method of hot rolling stainless steel through a plurality of passeswithout reheating which comprises insulating a stainless steel object bycovering the same with a relatively thick coating of a ceramicrefractory which is solid at the hot working temperature and heldtogether by a suitable hinder, the thickness of the coating being suchas to insulate the object sufficiently to hold the temperature of theobiect above approximately 2000 F. during rolling through the severalpasses and preferably of a thickness of about at least one eighth of aninch, heating the coated object to its hot rolling temperature of about2400 F. with the refractory thereon, and then rolling the heated objectto size by passing it through a plurality of passes.

References Cited in the file of this patent UNITED STATES PATENTS322,290 Hines July 14, 1885 1,080,059 Hatfield Dec. 2, 1913 1,085,768Thompson Feb. 3, 1914 1,432,523 Bickley Oct. 17, 1922 1,505,215lvieloche Aug. 19, 1924 1,879,701 Marino Sept. 27, 1932 1,922,005Stocking Aug. 8, 1933 2,121,606 McCulloch June 21, 1938 2,142,869 FraserIan. 3, 1939 2,430,083 Sherman Nov. 4, 1947 FOREIGN PATENTS 300,633Great Britain Nov. 28, 1929

